Oxetan-3-ones, preparation

This is one of the few methods available for the direct and efficient conversion of an acid, via the acid chloride, to an ortho ester. The preparation of the oxetane is straightforward, and a large number of oxetanes have been prepared [triol, (EtO)2CO, KOH]." In addition, the -butyl analogue has been used for the protection of acids. During the course of a borane reduction, the ortho ester was reduced to form a ketal. This was attributed to an intramolecular delivery of the hydride. ... 

The formation of thietanes from thiones and olefins has been less exploited for synthetic purposes than the corresponding oxetane-forming reaction. It should be remarked that thiocarbonyl compounds very often undergo efficient photoreactions from the second excited singlet state S2 U4). One interesting synthetic application is found in the photochemical preparation of quinolines from N-thioamides (4.84)498). The primary photochemical step is assumed to be the intramolecular thietane formation. 

The conditions for the photocycloaddition (discussed in detail in a later section of this review) can be relatively mild. There is usually a small probability of the oxetane being destroyed in dark reactions which would probably preclude isolation after preparation by any method. One mode of decomposition of oxetanes is fragmentation, either back to the starting materials or to the other possible carbonyl compound and olefin. For example, the oxetane from 4,4 -dimethoxybenzophenone and isobutylene forms readily and is easily detected and characterized by infrared and NMR spectroscopy. All efforts to purify it, however, have led to its decomposition into formaldehyde and the diarylethy-lene.17 37 In some cases, as with fluorenone and isobutylene37 or 2-methyl-2-butene,25b the oxetane is apparently too unstable for detection, but the presence of the olefin 96 attests to its formation. 

Polymers of these compounds are in widespread use. Poly-BCMO is a very tough, durable plastic, which is estimated to have an average lifetime of 70 years in water. As most oxetanes and /3-lactones can be polymerized and copolymerized to form a variety of useful solid compositions, this application will probably grow. One unusual application mentioned is the preparation and polymerization of 3,3-bis(azidomethyl)oxetane, which could be an explosive polymer (81MI51302). 

An attempt to extend this approach to the preparation of the oxetan-3-one system, i.e. use of an aldehyde in place of an epoxide, failed. Instead, a 2-acyl-3-substituted-l,4-dithiepane derivative was obtained. 

In the present review the ring systems containing one heteroatom are considered first, except for P-lactams which are given a special section at the end. Interest in azetidines continues to be stimulated by the discovery of the potentially useful trinitro derivative. The requirements for the stereoselective synthesis of substituted oxetane are being explored and derivatives of aluminium are useful in the stereoselective routes to oxetanones. The preparation and subsequent pyrolysis of oxetanones is suggested as an alternative to the Wittig route to olefins. Stereoselective routes to thietanes and thietane 1 -oxides are mentioned. 

The intramolecular nudeophilic substitution reaction - for example, the William-son-type reaction - represents one of the important methods for preparing oxetane ring structures, and have been widely applied to the synthesis of oxetanes (Scheme 7.1) [10]. Unfortunately, side reactions - which indude fragmentation from the intermediary alkoxide anion or elimination from the intermediary carboca-tion - often decrease the chemical yields of oxetane formation. 

The synthesis of fused oxetane derivatives via the Paterno-Bchi cycloaddition of carbonyls and alkenes is discussed in detail in a representative example is shown in Scheme 7 <2001CEJ4512>. See also Sections 3.3.1.8.3 and 3.4.1.10.5 for the preparation of fused oxetanes and azetdines by [2 + 2] cycloaddition reactions. Benzox-etan-2-one 10 has been prepared in an argon matrix by C02 loss from phthaloyl peroxide <1973JA4061>. 

The stereoselective synthesis of unsaturated oxetanes has recently been achieved by Feigenbaum and coworkers.Previous studies have indicated that photochemical cis-trans isomerization of enals is rapid and results in the formation of equivalent amounts of stereoisomeric alkene adducts. " For example, irradiation of rran.r-crotonaldehyde and 2,6-dimethylfuran produced a 1 1 mixture of alkenic isomers (174) and (175) in 64% yield. Irradiation of 4-trimethylsilylbutyn-2-one and furan provided with S 1 stereoselectivity the bicyclic oxetane (176) in which the methyl group occupies the exo position, presumably because of the small steric requirement of the triple bond. Desilyation of the protected al-kyne produced an alkynic oxetane which was hydrogenated under Lindlar conditions to bicyclic vinyl-oxetane (177) attempts to use the unprotected butyn-2-one gave low isolated yields of oxetane because of extensive polymerization. The stereochemical outcome thus broadens previous alkynyloxetane syn-theses and makes possible the preparation of new oxetane structures that may be synthetically useful. 

A C2-symmetric N,W-di-3,5-bis(trifluoromethyl)benzenesulfonyl-(ll ,21 )-l,2-diphenylethylenediamine 20-Et3Al complex promotes the [2-1-2] cycloaddition reaction between ketene and aldehydes to afford optically active 4-substituted oxetan-2-ones 21 (Scheme 20) [50]. The catalyst is prepared by mixing the bis-sulfonamide 20 and EtjAl, and the reaction proceeds by the coordination of the aldehyde to the chiral Lewis acid. 

A more practical method relies on the preparation of oxetane-3-ol and its subsequent oxidation to give oxetan-3-one (Scheme 13.3). " ° This procedure suffers from the necessity to use Pyridinium chlorochromate (PCC) for oxidation of oxetane-3-ol, because separation of oxetan-3-one from the reaction by-products could only be effected by preparative gas chromatography (GC). It was furthermore reported that other oxidizing conditions failed for this substrate. Having to rely on... 

Oxetane-2-ones are also y lactones [4]. They are prepared by cyclodehydration of y hydroxy-carboxylic acids with phenylsulfonyl chloride in pyridine ... 

Synthetic approaches to fluorinated p-lactones (oxetane-2-ones) are very similar to oxetane synthesis. Usually p-lactones are prepared either by cycloaddition of ketenes to aldehydes or ketones or by cyclodehydration of p-hydroxycarboxylic acids. 

---